Oil and other minerals are associated with specific types of underground formations. Hence, one method for locating potential drilling sites relies on mapping the formations. In this method, explosions are set off on the surface and the amplitude of the shock waves reflected from the various underground formations are recorded at a number of sites on the surface. The raw seismic data sets generated in such explorations typically require several hundred Gigabytes to store. Hence, these data sets are difficult to store and transmit. For example, satellite-based networking links to remote exploration sites have bandwidths from 9.5 Kilobits/sec to 128 Kilobits/secs. Hence, without some form of data compression, cost-effective, real time delivery of data to quality control and data processing centers remote from the exploration site is not feasible. Compression ratios between 10:1 and 100:1 are typically needed.
Compression systems can be divided into systems that exactly reproduce the original data after decompression, "lossless" systems and systems in which the decompressed data differs slightly from the original data. Historically, lossless systems have been used for seismic data compression. Unfortunately, lossless data compression systems are limited to compression ratios of less than 4:1.
Compression systems that make small errors in the decompressed data have been used extensively in image compression systems. Such systems can provide compression ratios in the range of 50:1. One compression system of this type is referred to as sub-band coding. In this system, the image is decomposed into a plurality of component images representing the data content of the original image in different ranges of spatial frequencies. The various component images are then coded with different precision depending on the amount of information contained in each component image. To the extent that one or more of these component images has a disproportionate share of the information in the image, image compression can be achieved. In the case of photographs, most images of interest to human observers have the relevant information concentrated in low spatial frequencies. Hence, such image compression systems provide high compression ratios with satisfactory image quality.
Broadly, it is the object of the present invention to provide an improved data compression system for seismic data.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.